One Rep Max Calculator

Your one-rep max (1RM) is the most weight you can lift for a single clean repetition — the benchmark for maximal strength. Rather than attempting a risky true max, this calculator estimates it from a set you can already do for several reps, using seven established prediction formulas.

How it works

Enter the weight you lifted and how many reps you completed — in kilograms or pounds; the calculator converts automatically and the formulas work the same either way. Each formula models the relationship between reps and load differently, so we compute all seven and average them, and show the range so you can see how much they agree:

Formula	Equation (w = weight, r = reps)
Epley	w × (1 + r/30)
Brzycki	w × 36 / (37 − r)
Lander	100w / (101.3 − 2.67123r)
Lombardi	w × r^0.10
Mayhew et al.	100w / (52.2 + 41.9·e^(−0.055r))
O’Conner et al.	w × (1 + r/40)
Wathan	100w / (48.8 + 53.8·e^(−0.075r))

If you enter a single rep, that weight is your 1RM, so the estimate returns it directly.

Worked example: bench and squat

Take the two lifts people ask about most. Say you bench press 100 kg (220 lb) for 5 reps. Epley’s formula — w × (1 + r/30) — gives 100 × (1 + 5/30) ≈ 117 kg (258 lb) as your estimated one-rep max. Run the same on a squat of 140 kg (309 lb) for 5 reps and it’s 140 × (1 + 5/30) ≈ 163 kg (359 lb). That single-formula sum is all there is to how to calculate a one-rep max by hand — the calculator just does it across all seven equations and averages them, so no one formula’s quirk skews your number.

Training-load table

Strength programs prescribe work as a percentage of 1RM. The calculator turns your estimate into concrete weights for common rep targets:

Reps	% of 1RM
1	100%
2	97%
3	94%
5	89%
8	81%
10	75%
12	69%

Getting an accurate estimate

Use a set taken close to failure in the 2–10 rep range — that’s where the formulas are most accurate. Rest fully beforehand, keep your form strict, and re-test every few weeks as you progress. Treat the number as a well-informed estimate, not a guarantee: always warm up and use a spotter before attempting a real maximal lift. And since strength is built on muscle, keep your protein intake high enough to support the training that pushes these numbers up.